In the field of automatic machining, it is often desirable to perform real time inspection of the machining operation and to terminate the operation if it appears that the machining process will not produce a part in accordance with a desired specification. By terminating machining operations prior to completion when a deviation from specification is detected, unnecessary wear of machine tools and costs associated with product waste and tie-up of manufacturing facilities and equipment are minimized. When machining some holes of small diameter, it is critical that the drill bore be true along a specified axis, and drilling diameter holes is often difficult due to stray of the drill from the desired axis. When working with small diameter drills which are flexible it would therefore be desirable to ascertain the location of the drill bit tip prior to ruining an expensive workpiece due to deviation of the bit from the specified bore axis.
Various techniques for determining the location of a drill bit are known in the geophysical arts and are employed in seismic applications. U.S. Pat. No. 3,828,867 to Elwood, teaches that a very low frequency electromagnetic transmitter may be disposed in a drill collar and sensors placed on the earth's surface around the well bore approximately in an equilateral triangle. Sensors receive very low frequency electromagnetic waves at frequencies of 5,000 hertz or below and the position of the drill bit is computed by triangulation.
None of these methods are particularly suited for precisely determining the location of a drill bit tip in a workpiece.